"Want preview bubbles for conversations in Gmail, as shown in the screenshot on the left? Then install the Gmail Conversation Preview Greasemonkey script. You can then right-click on any conversation to see its recent messages in a preview bubble. Greasemonkey 0.5 is required."

Quite possibly the Greasemonkey hack of the year. Unfortunately, it still has some major problems. After archiving a message, the script fails to properly track the inbox and frequently previews the wrong message. This can probably be remedied by avoiding the "Archive" button in preview mode. Hopefully, this will be corrected in a future version. In the meantime, it's stable and definitely worth a look. We'll be keeping an eye out for the next version.